Lake Shaokatan: A prairie lake with improving water quality

Plagued by toxic blue-green algal blooms for several years, the lake is now recording all-time lows of phosphorus, the nutrient that causes algae, and showing other signs of improvement, according to Steve Heiskary, research scientist with the Minnesota Pollution Control Agency (MPCA).

Typical of many shallow lakes in agricultural watersheds, Lake Shaokatan’s condition shows that long-term efforts can make a difference. This 995-acre lake near the town of Ivanhoe in Lincoln County has a maximum depth of 10 feet. It has a ratio of 9 acres of drainage area for every 1 acre of water.

Excessive nutrient runoff from neighboring farm fields and developed shorelines likely led to extensive algae blooms in the lake. The lake has a history of water quality problems including severe nuisance blue-green blooms, low oxygen levels in summer and winter, and periodic fish kills. These problems were the result of excessive amounts of nutrients flowing into the lake and settling into the bottom.

Lake Shaokatan was part of a Clean Water Partnership effort, sponsored by the Yellow Medicine Watershed District, involving the MPCA, state and federal agencies, local groups, and local units of government. A detailed diagnostic study started in 1991 and restoration efforts were underway by 1993.

These efforts included rehabilitation of three animal feedlots, four wetland areas, and shoreline septic systems. These actions resulted in a 58 percent reduction in phosphorus loading into the lake. By 1994, phosphorus levels in the lake dropped significantly, with concentrations near the state standard of 90 parts per billion, down from 200 to 350 parts per billion in previous summers. This decrease resulted in reductions in the frequency and severity of nuisance algal blooms.

In addition, water clarity increased and residents reported that rooted plants — instead of algal plants — were increasing.

However, plant surveys in 2000 and 2002 found essentially no rooted plants. Also, from 1999-2001, phosphorus and chlorophyll-a — the pigment that makes algae green — were increasing, according to water monitoring data. These increases were largely attributed to a major runoff event from an abandoned feedlot in the shoreland area of the lake. Subsequent efforts by the Yellow Medicine Watershed District, Lincoln County and the local sportsmen’s group sought to address the problem.

Despite these efforts, phosphorus and chlorophyll-a levels still violated water quality standards. The MPCA officially listed the lake as impaired in 2002, requiring a Total Maximum Daily Load study to determine the maximum amount of nutrients the lake can accept and still meet standards. That study led to further restoration efforts that are now paying off.

In 2008, the MPCA included Lake Shaokatan in another long-term monitoring program, Sustaining Lakes In a Changing Environment (SLICE). SLICE lakes are monitored more often to assess physical, chemical, and biological characteristics, and to gauge the current health of habitats and fish in Minnesota lakes.

This monitoring has been valuable for tracking improvements in the condition of Lake Shaokatan:

Phosphorus concentrations in 2013 and 2014 registered some all-time lows for the lake, well below the impairment level (chart below). These decreases resulted in less frequent algal blooms.

Water clarity averaged 5.6 feet (1.7 meters) in both 2013 and 2014.

In 2014, chlorophyll-a remained below 20 parts per billion (a level indicative of nuisance blooms) all summer and averaged 9 parts per billion — a stark contrast to typical summer-means of 60-70.

In 2014, there was also a shift in the composition of the algae, with diatoms and other forms present on all sample dates, in contrast to most summers when the blue-greens Anabaena and Aphanizomenon were the dominant forms throughout most of the summer.

Rooted plants were evident across the lake in 2014, based on observations by MPCA staff and shoreline property owners

“It is too early to tell if the 2014 phosphorus and chlorophyll-a concentrations will become the new norm for Shaokatan. However, continued monitoring in 2015 through SLICE will provide an opportunity to track changes in the condition of this lake,” Heiskary said.